Commutating brush



Aug. 30, 1966 L. J. G. BURSKI COMMUTATING BRUSH Filed July 17, 1963INVENTOR. ,5 6 Leonard JG. Burski Deceased Dorothy ae Schlemmer- Burski8y Execu'h'ix HTTDR'VEYS r: 3,270,306 Patented August 30, 1966 3,270,306(IOMMUTATING BRUSH Leonard J. G. Burslri, deceased, late of Bristol,Conn, by Dorothy Mae Schlemmer Burslci, executrix, Bristol, Conn.,assignor to The Superior Electric Company, Bristol, Conn, a corporationof Connecticut Filed July 17, 1963, Ser. No. 295,850 2 Claims. (Cl.336148) The present invention relates to a cornmutating brush thatmaintains a slidable, electrical contact with a commutator surface andmore particularly to such a brush that is advantageously employed in anadjustable voltage autotransformer having'a commutator surface formed ofseparate portions with a voltage potential existing between eachportion.

One common design of an adjustable voltage autotransformer that isgenerally commercially available includes a toroidal winding wound on anannular iron core with a portion of each turn of the winding being baredtoform a commutator surface. A commutating brush is slidable on thesurface and is in electrical engagement therewith so that movement ofthe brush along the surface provides for adjusting the output voltage ofthe autotransforrner. Such an autotransformer is disclosed in theKarplus et al. Patent No. 2,009,013 and in said patent there is setforth the requirement that there be a resistance interposed in theoutput circuit of the autotransformer that while being a hindrance tothe output current, is necessary in order to limit the short-circuitcurrent that occurs when the brush is spanning the commutating portionof adjacent turns of the winding as the potential between the turnscauses the short-circuit current to pass from one turn through the brushto the other turn. The resistance as taught in the above-noted patentand in comercially available autotransformers, should be a contactresistance that exists between a graphitic brush and the commutatorsurface but while limiting the short-circuit current the contactresistance also has the output current flowing therethrough and thus isproduces heat at the contact between the brush and commutator surface.As the heat is very localized and difficult to dissipate, itsubstantially causes the temperature of the brush and commutator surfacecontacted by the brush to achieve such a value that the temperatureplaces a limitation on the power handling capability of theautotransformer as a maximum temperature should not be exceeded at thebrush and commutator surfaces irrespective of the ability of theautotransformer without such brush heating to have a higher powerhandling capability.

It is accordingly an object of the present invention to provide acommutating brush that limits short-circuit current without unduehindrance to an output current but which substantially minimizes thelocalized heat produced at the contacting surfaces of the brush and thecommutator surface and thus the temperature thereat.

Another object of the present invention is to provide a commutatingbrush of the above type which while having the necessary resistance tolimit short-circuit current, broadly spreads the heat developed by theresistance so that it may be easily dissipated.

A further object of the present invention is to provide a commutatingbrush that is simple in construction, reliable in use and relativelydurable.

A feature of the present invention resides in forming a commutatingbrush from a plurality of individual electrical conducting wires, eachwire being formed of metal and having a relatively small diameter. Aterminal portion of each of the wires is held in parallel alignment andthe end surface of each wire is made to provide a substantially planarcontact surface. The brush, in use, has its planar end surface slidableon and in electrical engagement with the commutator surface and as thewires and the commutator surface are of conducting metal, the contactresistance therebetween is substantially minimized. However, in order toinsert the required shortcircuit resistance in the turns of the windingthat are spanned by the brush, the metal of the wires is selected tohave a specific resistance which is sufficient, depending upon theirlength, to provide the required resistance. Accordingly, the heatproduced by the resistance is spread throughout the length of the wires.

In order to prevent conduction between the wires, the present inventionprovides for electrically insulating each wire from the other except attheir other end, the planar surface being the first end where there is acommon connection, by employing wires having an insulating covering.Thus output current flows throughout the length of each wire whileshort-circuit is forced to flow twice the length of each wire, therebyin effect doubling the resistance of the brush to short-circuit currentas compared to its resistance to the output current.

Other features and advantages will hereinafter appear.

In the drawing:

FIGURE 1 is a plan view of a brush construction according to the presentinvention with the wires being shown substantially enlarged for clarity.

FIG. 2 is an end view taken along the line 2--2 of FIG. 1.

FIG. 3 is an elevation, partly in section and having a magnifiedportion, of an adjustable voltage autotransformer in which the brush ofthe present invention is advantageously incorporated.

FIG. 4 is a plan of the autotransformer of FIG. 3 with portions brokenaway, also having a magnified section.

FIG. 5 is a modification of the brush of the present invention with thewires again being shown substantially enlarged.

FIG. 6 is a further modification of the brush of the present invention.

Referring to the drawing, the commutating brush of the present inventionis generally indicated by the reference numeral 10 and is shown employedin an autotransformer 11. The autotransforrner, as disclosed in theabove-mentioned Karplus et al. patent includes a winding 12 of insulatedwire toroidally wound about an annular magnetic iron core (not shown)with a commutator surface 13 formed on an axial end of the winding byremoving insulation 14 and flattening as by grinding, a common portion15 of each of the turns 16 of the winding. It will be appreciated that,as shown in FIG. 3, each of the portions 15 is separated slightly fromadjacent portions so that a gap 13a exists therebetween. Moreover, as iswell known, a potential difference exists between each of the portions15 when the autotransformer is energized.

The brush 10 is mounted in a brush holder 10a of any convenient designprovided on a radiator disk 17 that is secured to a rotatable shaft 18so that current may be conducted from the commutator surface to thebrush, to the radiator and through appropriate slip ring connections toan output terminal (not shown) as is Well known in the art. It will thusbe seen that, by rotation of the shaft 18 which carries the radiatordisk and the brush, the brush is caused to slide in electricalengagement over the commutator surface 13 formed by the portions 15 andby so doing adjust the output voltage of the autotransforrner.

The brush of the present invention, as shown in FIG. 1, includes aplurality of lengths of wire 19 of substantially equal lengths, eachhaving a terminal portion 20 with the portions 20 being held in parallelalignment and with the end 21 of each wire being positioned to form asubstantially planar surface 22. From the terminal portions 20,

the plurality of wires 19 are separated into two groups 23 and 24 witheach group terminating in a connecting clip 25 and 26 respectively. Theclips 25 and 26 are thus electrically connected to the other ends 27 ofthe Wires. As shown in FIG. 2, each of the wires preferably has aninsulating covering 28 in order to prevent conduction therebetweenexcept at the ends 27. The terminal portions in the embodiment shown inFIGS. 1 and 2 are encased by a hardened material 29, such as an epoxyresin having the shape shown, in order to hold them in parallelalignment and to form the terminal portions into a rigid structure whichmay be clamped to the radiator disk 17. Moreover, as shown in FIG. 1,the end. 21 of each wire is exposed and located in the planar surface 22and if desired may project a slight distance beyond its insulation 28and encasing material 29. But normally the insulation extends to theplanar surface to thus provide rigidity for the wire ends.

The diameter of each of the wires is substantially the same andpreferably relatively small, on the order of .003 inch. While it iscontemplated that wires having different diameters may be employed, inany event the diameter of each wire must be less than the gap 13a thatexists between each of the portions 15 of the commutator surface so thatone wire 19 is incapable of simultaneously being in contact with twocommutating portions 15.

In carrying out the present invention, wherein it is required that therebe a resistance to limit short-circuit current, as taught by the Karpluset al. patent, each wire is formed of a material that is selected tohave a specific resistance that provides the necessary value ofresistance. Thus, for example, high brasses may be used or otherresistive metals which by reason of their diameter, specific resistanceand length provide the necessary value of resistance. It is presentlypreferred to make each wire identical as to length, material anddiameter so that the resistance of each wire will 'be the same and hencethe wires will conduct current evenly.

It will be appreciated that the planar surface 22 of the brush thatslides on the commutator surface 13 is metallic as are the portions thatform the commutator surface and hence by reason of the metal to metalcontact, there is minimum contact resistance between the commutatorsurface and the brush. As each wire 19 that carries current has aresistance, with the resistance being constant throughout the length ofthe wire, then the heat developed by the resistance is spread throughoutthe length of the wire. This feature of the present invention isadvantageously employed when the brush is utilized in an adjustablevoltage autotrans-former where, as shown in FIG. 3, the brush 10 isclamped in the radiator disk to engage the commutator surface 13 and thetwo groups 23 and 24 are positioned within circular Ways 30 formed onthe radiator disk 17 by upstanding arcuate heat dissipating flanges 31.Thus, with the thermal contact between the wires and the radiator disk,the heat from the wires is easily conducted to the radiator disk fordissipation. As in conventional autotransformer construction theradiator is also employed as a conducting member and hence the clips aresecured thereto as at 32.

The encasing compound is preferably insulating or poorly conductingmaterial or material having a high contact resistance drop and moreoverit is preferred and advantageous if it has a poorer wearing quality thanthe metal of the wires 19. Thus for example nylon, carbon with aresinous binder or graphite with an epoxy binder may be employed.Moreover, if desired, the encasing material may be impregnated with alubricant, such as oil, to provide lubrication of the commutatorsurface. The insulating covering 28 of the wires 19 is preferably aVarnish, such as that sold under the trade name Formvar, which also haspoor wearing qualities.

While the embodiment of the present invention shown in FIG. 1 has beenfound satisfactory, in some instances the amount of heat produced andthe heat conduction between the brush and the radiator disk may be suchthat the length of the wires required to dissipate the heat may bematerially shortened. Thus, as shown in FIG. 6, a brush 33 in thisembodiment comprises, in essence, just short lengths of wire 34 with acontact member 35 attached to the radiator disk 36 engaging the upperend 37 of the brush 33 for electrical conduction therebetween. The brushherein is, in essence, identical in construction to the terminal portionof the brush 10 shown in FIG. 1 and is clamped to the radiator disk sothat the radiator supports the brush 33 and also serves to conduct heattherefrom. In this embodiment, it will also be appreciated that the heatdissipated by the resistance of the brush is located throughout thelength of the wires forming the brush and not just at the contactsurfaces between the brush and the commutator surface.

While both embodiments of the present invention heretofore describedhave been shown as having the terminal portion encased in a hardenedcompound, in the embodiment shown in FIG. 5 a rigid clamp consisting ofa substantially U-shaped member 38, a closing member 39 and screws 49may instead be employed to maintain the terminal portions of the wiresin aligned parallel relation. Preferably the members 38 and 39 are ofheat conducting metal.

It will thus be seen that the present invention of a commutating brusheliminates the localized heating that occurs at the brush and.commutator surface in the heretofore taught concept of having theresistance for limiting short-circuit current be at the contactingsurfaces. The present invention minimizes, if not substantiallyeliminates, the contact resistance between the brush and the commutatorsurface and employs the resistance of lengths of conducting wires tospread the heat created by the resistance throughout the length of thewires. In some embodiments the lengths of the wire may, in order toprovide adequate spreading of the heat, be of substantial length whilein others small lengths may be sufiicient. While the brush heretoforedisclosed for use in an autotransformer has just one terminal portion,the present invention where larger brushes may be required alsocontemplates making a plurality of separate small terminal portionsinstead of one large terminal portion.

It will be appreciated that the heretofore disclosed commutating brushmay be advantageously employed in an adjustable voltage autotransformer,for obviating the limitation of the power handling capacity of anautotransformer because of the maximum temperature occurring at thecontacting surfaces, thus enabling an autotransformer to handle morepower. Moreover, by reason of the number of lengths of Wire in a brush,for example 500, even though the autotransformer is handling arelatively large amount of current, for example 50 amperes, the amountof amperes per wire is accordingly small and hence sparking or arcing atthe commutator surface is substantially descreased.

Variations and modifications may be made within the scope of the claimsand portions of the improvements may be used without others.

What is claimed is:

1. In combination with an adjustable voltage autotransformer having acommutator surface formed of a plurality of separate commutatingsegments with a spacing between adjacent segments and with an electricalpotential existing between adjacent segments, the improvement comprisinga commutating brush formed of a substantial number of separateelectrical conducting wires, each Wire having a first end and a firstterminal portion and a second end and a covering of electricalinsulating material with the ends being free of insulating material,means for maintaining the terminal portions of the wires substantiallyrigid to have the first ends form a substantially planar electricallyconducting surface free of insulation, means for electrically connectingtogether the second ends of the wires to a common connector, means formounting the brush on the autotransformer to have the planar surfaceslidable on and in electrical engagement with the commutator surface,said planar surface being of a width to span at least two segments withthe first end of each wire having a width less than the width of thespacing, said wires being dimensioned and formed of electricalresistance material to have the resistance of the Wires between the endssubstantially larger than the contact resistance between the first endsand the commutating surface whereby the passage of current through thewires generates substantially more heat along the length of the wiresthan at the engagement between the first ends and the commutatingsurface.

2. The invention as defined in claim 1 in which the means for mountingthe brush includes a heat radiator References Cited by the ExaminerUNITED STATES PATENTS 276,233 4/1883 Edison 310-248 X 345,336 7/1886Thomson 31()248 X 3,025,452 2/1958 Ross 336-149 X LARAMIE E. ASKIN,Primary Examiner.

JOHN F. BURNS, Examiner.

C. TORRES, Assistant Examiner,

1. IN COMBINATION WIHT AN ADJUSTABLE VOLTAGE AUTOTRANSFORMER HAVING ACOMMUTATOR SURFACE FORMED OF A PLURALITY OF SEPARATE COMMUTATINGSEGMENTS WITH A SPACING BETWEEN ADJACENT SEGMENTS AND WITH AN ELECTRICALPOTENTIAL EXISTING BETWEEN ADJACENT SEGMENTS, THE IMPROVEMENT COMPRISINGA COMMUTATING BRUSH FORMED OF A SUBSTANTIAL NUMBER OF SEPARATEELECTRICAL CONDUCTING WIRES, EACH WIRE HAVING A FIRST END AND A FIRSTTERMINAL PORTION AND A SECOND END AND A COVERING OF ELECTRICALINSULATING MATERIAL WITH THE ENDS FREE OF INSULATING MATERIAL, MEANS FORMAINTAINING THE TERMINAL PORTIONS OF THE WIRES SUBSTANTIALLY RIGID TOHAVE THE FIRST ENDS FORM A SUBSTANTIALLY PLANAR ELECTRICALLY CONDUCTINGSURFACE FREE OF INSULATION, MEANS FOR ELECTRICALLY CONNECTING TOGETHERTHE SECOND ENDS OF THE WIRES TO A COMMON CONNECTOR, MEANS FOR MOUNTINGTHE BRUSH ON THE AUTOTRANSFORMER TO HAVE THE PLANAR SURFACE SLIDABLE ONAND IN ELECTRICAL ENGAGEMENT WITH THE COMMUTATOR SURFACE, SAID PLANARSURFACE BEING OF A WIDTH TO SPAN AT LEAST TWO SEGMENTS WITH THE FIRSTEND OF EACH WIRE HAVING A WIDTH LESS THAN THE WIDTH OF THE SPACING, SAIDWIRES BEING DIMENSIONED AND FORMED OF ELECTRICAL RESISTANCE MATERIAL TOHAVE THE RESISTANCE OF THE WIRES BETWEEN THE ENDS SUBSTANTIALLY LAGERTHAN THE CONTACT RESISTANCE BETWEEN THE FIRST ENDS OF THE COMMUTATINGSURFACE WHEREBY THE PASSAGE OF CURRENT THROUGH THE WIRES GENERATESSUBSTANTIALLY MORE HEAT ALONG THE LENGTH OF THE WIRES THAN THEENGAGEMENT BETWEEN THE FIRST ENDS AND THE COMMUTATING SURFACE.